Malia Akutagawa, University of Hawaiʻi at Mānoa, Hawaiʻinuiākea School of Hawaiian Knowledge, Kamakakūokalani Center for Hawaiian Studies, William S. Richardson School of Law, Ka Huli Ao Center for Excellence in Native Hawaiian Law

Rosie Alegado, University of Hawaiʻi at Mānoa, Department of Oceanography, UH Sea Grant

Tiffany Anderson, University of Hawai‘i at Mānoa, Geology and Geophysics

Patrick Barnard, U.S. Geological Survey–Santa Cruz

Rusty Brainard, NOAA Pacific Islands Fisheries Science Center

Laura Brewington, East-West Center, Pacific RISA

Jeff Burgett, Pacific Islands Climate Change Cooperative

Rashed Chowdhury, NOAA Pacific ENSO Applications Climate Center

Makena Coffman, University of Hawai‘i at Mānoa, Urban and Regional Planning

Hawai‘i and U.S.-Affiliated Pacific Islands

Coastal Communities and Systems

The majority of Pacific island communities are confined to a narrow band of land within a few feet of sea level. Sea level rise is now beginning to threaten critical assets such as ecosystems, cultural sites and practices, economies, housing and energy, transportation, and other forms of infrastructure. By 2100, increases of 1–4 feet in global sea level are very likely, with even higher levels than the global average in the U.S.-Affiliated Pacific Islands. This would threaten the food and freshwater supply of Pacific island populations and jeopardize their continued sustainability and resilience. As sea level rise is projected to accelerate strongly after mid-century, adaptation strategies that are implemented sooner can better prepare communities and infrastructure for the most severe impacts.

The rate of global average sea level rise has accelerated102,103 and has become very damaging in the region (Figure 27.8). Impacts include coastal erosion,7,8 episodic flooding,9,10 permanent inundation,11 heightened exposure to marine hazards,12 and saltwater intrusion to surface water and groundwater systems.13,14 Already apparent on many shorelines, these problems endanger human communities by negatively impacting basic societal needs, such as food and freshwater availability, housing, energy and transportation infrastructure, and access to government services.104

Sea level could rise by as much as 1 foot by 2050 and by as much as 4 feet by 2100. Emerging science suggests that, for the Extreme sea level rise scenario, sea level rise of more than 8 feet by 2100 is physically possible. It is extremely likely that sea level rise will continue beyond 2100.17,105

Communities in Hawaiʻi and the USAPI typically live in low-lying settings clustered around the coastal zone. Whether on high volcanic islands or low reef islands (atolls), exposure to marine hazards and dependency on global trade mean escalating vulnerability to climate change (Ch. 16: International, KM 1).18

Roadways Flood Periodically on Oʻahu

Figure 27.8: The photo shows North Shore, Oʻahu, in the winter of 2016. Episodic flooding in the Pacific Islands …

Until recently, global sea level rise of about 3 feet by the end of the century was considered a worst-case scenario, becoming more likely without reductions in global greenhouse gas emissions.106 However, new understanding about melting in Antarctica,107,108,109 Greenland,110 and alpine ice systems;111 the rate of ocean heating;112,113 and historical sea level trends103 indicates that it is physically possible to see more than double this amount this century (see also Ch. 2: Climate, KM 4).17,114

The Intermediate sea level rise scenario predicts up to 3.2 feet of global sea level rise by 2100; however, recent observations and projections suggest that this magnitude of sea level rise is possible as early as 2060 in a worst-case scenario.17 Studies in Hawaiʻi show that the value of all structures and land projected to be flooded by 3.2 feet of sea level rise amounts to more than $19 billion (in 2013 dollars; $19.6 billion in 2015 dollars) statewide (Figure 27.9).42 Across the state, nearly 550 Hawaiian cultural sites would be flooded or eroded, 38 miles of major roads would be chronically flooded, and more than 6,500 structures and 25,800 acres of land located near the shoreline would be unusable or lost, resulting in approximately 20,000 displaced residents in need of homes.42

Owing to global gravitational effects, sea level rise will disproportionately affect the tropical Pacific15 and potentially exceed the global average.16 This, plus sea level variability internal to the Pacific Basin (see Figure 27.3), means that parts of the region are likely to experience the highest rates of sea level rise on the planet.115 Scientific understanding of the timing and magnitude of future global sea level rise continues to improve,116,117 making regular updates of management plans and engineering codes an important activity for island communities.

Figure 27.9: This map highlights potential economic losses (in 2015 dollars) in the exposure area associated with 3.2 feet of sea level rise on the island of O‘ahu, Hawai‘i. Potential economic losses are estimated from impacts to land and residential and commercial infrastructure. Highly impacted areas at risk of large economic losses include the U.S. Pacific Command and military infrastructure concentrated in Pearl Harbor (black circle) and the vulnerable tourist areas surrounding Waikīkī (dashed black circle). Source: adapted by Tetra Tech Inc. from the Hawai‘i Climate Change Mitigation and Adaptation Commission 2017.42

Because of accelerating sea level rise, coastal communities are projected to experience saltwater intrusion of aquifers and agricultural resources. As sea level rise continues in coming decades, freshwater sources will become increasingly at risk in communities dependent on restricted groundwater supplies.69 Saltwater intrusion, which is amplified by climate variability and changing precipitation patterns (see Key Message 1),12 is difficult to prevent, and, once damaged, water and food resources are challenging to restore.13

Future changes in global and regional precipitation vary among current climate models,34,35,118 but the potential for changes in precipitation and the projected impacts of saltwater intrusion cast uncertainty over the sustainability of freshwater resources throughout the region. Because many island groups are very isolated, severe drought punctuated by saltwater intrusion can displace communities and produce feedback effects, such as failure of cultural, health, education, and economic systems (Ch. 17: Complex Systems).119 However, strategic planning for the inevitability of these events can greatly reduce their impact.

In many areas, Pacific island coastal populations already exist on the edge of sustainability. Urban areas typically cluster around port facilities, as nearly all Pacific communities are tied to goods and services delivered by cargo ships. As the world’s most isolated chain of islands, Hawaiʻi imports nearly 90% of its food at a cost of more than $3 billion per year (in 2004–2005 dollars),120 resulting in government programs focused on food security.121 Without adaptation measures, the additional stress on sustainable practices related to sea level rise is likely to drive islanders to leave the region and make new homes in less threatened locations (see Key Message 6 and the Case Study “Bridging Climate Science and Traditional Culture”).122

Away from urban areas, many island communities rely on food gathered from the ocean and land. Populations on remote reef islands throughout Micronesia depend on water, food, and medical assistance that are often in question and are a source of persistent community stress. Extreme water levels accompanied by high waves have swept over remote atoll communities and destroyed taro patches, contaminated fragile aquifer systems, and deeply eroded island shores.9,10,58

In 2007, extreme tides coupled with high waves flooded the Federated States of Micronesia and triggered a national emergency. Food, water, and medical supplies had to be immediately delivered to dozens of communities in widely distributed locations to prevent famine (see Key Message 1)(see also Ch. 14: Human Health, KM 1).57 It is likely that events of this type will increase in frequency as sea level rise accelerates in the future.

Rising sea surface temperatures are shifting the location of fisheries (Ch. 9: Oceans, KM 2).123 Ocean warming 124 and acidification,125,126 coupled with damaging watershed127 and reef practices,128 converge on island shores to increasingly limit the food resources that can be gathered from the sea (see Key Message 4).129 Growing exposure to coastal hazards, such as storm surges,130 compounds this threat to sustainability.

The Pacific Ocean is highly variable; fundamental characteristics of ENSO (see Box 27.1) appear to be changing.131 Both El Niño and La Niña episodes are projected to increase in frequency and magnitude as the world warms.3,52 Patterns of variability are complex,132,133 and as climate changes over the long term, the oceanic and atmospheric forces that cause shorter-term climate variability (such as ENSO) also will be changing. Model projections indicate changing future wave conditions that will vary in complex ways spatially, by season, and with shoreline exposure and orientation.37,38,134 These changes will challenge community efforts to define adaptation plans and policies.

The 2015–2016 El Niño was a Pacific-wide event with widespread impacts.135 As warm water shifted from west to east, Palau, Yap, and other western Pacific communities experienced deep drought, requiring water rationing, as well as falling sea level that exposed shallow coral reefs.136 In the central Pacific, Hawaiʻi experienced 11 days of record-setting rainfall that produced severe urban flooding,137 while American Sāmoa faced long-term dry conditions punctuated by episodic rain events. Honolulu experienced 24 days of record-setting heat that compelled the local energy utility to issue emergency public service announcements to curtail escalating air conditioning use that threatened the electrical grid (Ch. 4: Energy, KM 1).138 Nine months of drought stressed local food production, and a record tropical cyclone season saw Hawaiʻi monitoring three simultaneous hurricanes at one point.139

There is great uncertainty about how Pacific variability occurring on shorter timescales (for example, El Niño and La Niña) will combine with multidecadal changes in temperature, waves, rainfall, and other physical factors. This variability affects sea level extremes, which are likely to become more frequent this century,4,12 along with changes in precipitation,140 ocean temperature,113 and winds.141 These, in turn, drive difficult-to-forecast stressors that challenge the sustainability of coastal communities.

To date, tropical cyclone frequency and intensity have not been observed to change in the region of the USAPI. Trade winds and monsoon wind characteristics are expected to change in the future, but projections for specific geographic locations are unclear.142 Under scenarios with more warming (for example, SRES A1B),143 wind speeds are projected to decrease in the western Pacific and increase in the South Pacific;142 central Pacific tropical cyclone frequency and intensity are expected to increase;40,142 and in the western and South Pacific, tropical cyclone frequency is projected to decrease, while cyclone intensity is projected to increase.142 Combined with continued accelerations in sea level rise, storm surge associated with a tropical cyclone has the potential to deliver a profound shock to a community beyond any ability to meaningfully recover.

Adaptation. Despite these threats, many Pacific communities are growing more resilient with renewed focus on conservation,144 sustainably managing natural resources,145 adapting to climate change,146 and building more resilient systems.147 Pacific island governments are taking steps to anticipate marine flooding (securing food and water resources) and doing so in the context of environmental conservation. Islanders throughout the USAPI are committing to demonstrate climate leadership, identifying sector vulnerabilities, and calling on their international partners to support their implementation of climate change resilience and adaptation actions.55,148,149,150,151,152